Before inoculation into products, such as food products, bacteria are cultured in order to provide a suspension containing large amounts of bacteria. The suspension is usually concentrated using centrifugation, filtration, distillation, sedimentation or flocculation. This concentration step is often followed by freezing or freeze-drying or drying or storage of the microbial concentrate as a frozen product in liquid nitrogen to preserve and/or store the bacteria.
In addition to providing a concentrate of desired bacteria, the concentration step also reduces the volume of the suspension to be treated for preservation and/or storage. By reducing the volume of the suspension and increasing the concentration of the bacteria in the suspension, cost reduction advantages are obtained in downstream processing steps (for example cost reduction of freezing, freeze-drying, drying and transportation). In an industrial scale, the concentrate must remain sufficiently flowable to permit further processing.
However, preservation (such as freezing or freeze-drying) of the bacteria is a bottleneck in the industrial production of storable viable bacteria due to the cell damage and loss of viable cells during the freeze-drying but also due to the long process time and the high costs that are associated with industrial freeze drying processes. Moreover, the amount of water remaining after drying affects not only viability of lactic acid bacteria, as determined immediately after the process, but also the rate of loss of viability during subsequent storage (de Valdez et al. (1985), Appl Environ Microbiol. 49(2):413-15). The initial water activity (aw) of freeze-dried products is very important for the cell survival as higher water activity in the freeze-dried products affects the viability of the bacteria during storage; thus the bacteria are more sensitive to the environmental stresses during storage (Ferreira et al. (2005), Biotechnol Lett, 27(4):249-52). Also, the water activity (aw) is an important factor affecting the stability of dried products and it is used as a critical control point for Hazard Analysis and Critical Control Points (HACCP) programs.
Therefore, there is still a need to improve the efficiency of purification and concentration methods suitable for bacteria-containing suspensions, to obtain a highly concentrated bacteria suspension, more efficient freeze-drying process and a limited loss of activity (i.e., a limited loss of viable bacteria). These methods need to be feasible at any scale, but especially on the industrial scale, where large volumes of suspension are concentrated.